The present invention generally relates to a projection-type color display device and, in particular, to a projection-type color display device which uses a plurality of light valves for forming picture images.
There are several versions of projection-type color display devices which utilize a light valve. For example, U.S. Pat. Nos. 4,461,542 and 4,425,028 disclose a color display device in which a reflecting light valve and dichroic mirrors synthesize and project monochromatic pictures. A color display device which uses an oil-membrane light valve is disclosed in an article entitled "Matrix-Addressed Liquid Crystal Projection Display" published in the 1972 Society for Information Display (SID) at pp. 62-63, and in an article entitled"Recent Advances in the Single-Gun Color Television Light-Valve System" published in the 1975 SID at pp. 24-27. An article on the relevant technology entitled"Optical Properties of a Liquid-Crystal Image Transducer at Normal Incidence: Mathematical Analysis and Application to the Off-State" can be found in J.Opt.Soc.Am., Vol 70, No. 3, March 1980 beginning at page 287.
The conventional projection-type color display devices have several deficiencies. In a device using a reflecting light valve: first, the reflected light at the surface of the light valve causes deterioration of the contrast of the displayed images; second, since the light valve is addressed by the light from a cathode ray tube (CRT), the device inevitably becomes large and complex, and; third, there is the requirement for an excellent polarized light dividing characteristic as well as a colored light dividing characteristic of the dichroic mirror.
In the second device using an oil-membrane light valve, the device is large, complex and expensive and is less than satisfactory with respect to its life span and utilizing efficiency of the light.
Accordingly, the present invention was developed to eliminate the problems in the prior art as described above and to provide a small-sized projection-type color display device which is excellent in the contrast of the displayed pictures and in the utilizing efficiency of the light from the light source.